flight beyond Earth’s atmosphere. This article deals with the basic concepts associated with the launch and return of unmanned and manned spacecraft and their travel, navigation, and rendezvous and docking in space. For the development of space travel and discussions of spacecraft and space programs and their contributions to scientific knowledge and human welfare, see space exploration. For the development and technology of rocket propulsion, see rocket. For details on rocket systems used to propel spacecraft beyond Earth’s atmosphere, see launch vehicle. The space environment Space, as considered here, is defined as all the reaches of the universe beyond Earth’s atmosphere. There is no definitive boundary above Earth at which space begins, but, in terms of the limiting altitude for vehicles designed for atmospheric flight, it may be considered to be as low as 45 km (28 miles). The lowest practical orbit for an artificial satellite around Earth is about 160 km (100 miles). By...